FIG. 1 depicts an example of a configuration of a radio communication system of LTE (long term evolution).
The radio communication system 100 depicted in FIG. 1 includes, for example, a plurality of radio base stations (eNode B, referred to below as eNB) 300-1 to 300-3 capable of communicating with a radio terminal as an example of UE (user equipment) via a wireless link, and MME (mobility management entity)/SGW (serving gateway) 200-1 and 200-2, which are higher level devices to the eNBs 300-1 to 300-3. When the eNB 300-1 to the eNB 300-3 are not distinguished, they are referred to below as the eNB 300. When the MME/SGW 200-1 and the MME/SGW 200-2 are not distinguished, they are referred to as the MME/SGW 200.
In the above radio communication system 100, communication between the eNB 300 and the MME/SGW 200 is performed via an inter-device interface called the S1 interface. On the other hand, communication between the eNBs 300 is performed via an inter-device interface called the X2 interface.
The S1 interface is used to connect between the eNB 300 and the MME/SGW 200 using the IP (Internet protocol) to transfer a signal of a control plane (C-plane) and/or a user plane (U-plane). On the other hand, the X2 interface is used to connect between the eNB 300-1, eNB 300-2, and eNB 300-3 using the IP to transfer a signal of a control plane and/or a user plane.
One communication protocol used when a control signal is transferred in the S1 interface, X2 interface, or the like is SCTP (Stream Control Transmission Protocol).
SCTP is a transport layer communication protocol that avoids duplicate transmission, loss, or the like of a packet as much as possible and enables reliable information transfer by validating a packet on the basis of a sequence number and checksum over the IP.
A network entity (also referred to below as a node) with the SCTP function has one or more of logical terminal points called endpoints and establishes a logical connection (SCTP link), which is called an association, with an endpoint of another node. At this time, a node (endpoint) is in one of two states: a client and server. A node in the state of a client is requested to operate as a transmission side of a request for establishing a connection (association) and a node in the state of a server is requested to operate as a reception side of the request.
A packet in SCTP includes an SCTP common header and one or more of data blocks called chunks that follow the SCTP common header. There are two types of chunks: a control chunk in which a control signal (message) is stored and a data chunk in which user data is stored. A control chunk is transmitted when, for example, an association is established (initialization: INIT) or released.
For example, when an association is established, an INIT chunk, INIT-ACK chunk, COOKIE-ECHO chunk, COOKIE-ACK chunk, or the like is used as a control chunk. On the other hand, when an association is released, a SHUTDOWN chunk, SHUTDOWN-ACK chunk, SHUTDOWN-COMPLETE chunk, or the like is used as a control chunk.
Next, an example of a protocol stack of the control plane of the S1 interface in LTE is depicted in FIG. 2 and an example of a protocol stack of the control plane of the X2 interface in LTE is depicted in FIG. 3.
As depicted in FIG. 2, in the S1 interface, the physical (PHY) layer, data link (MAC: media access control) layer, IP layer, SCTP layer, and S1-AP (application) layer are defined, in order from the lowest layer, as the protocol stack of the control plane. On the other hand, as depicted in FIG. 3, in the X2 interface, the physical (PHY) layer, data link (MAC) layer, IP layer, SCTP layer, and X2-AP (application) layer are defined, in order from the lowest layer, as the protocol stack of the control plane.
In Internet networks, TCP (transmission control protocol) and UDP (user datagram protocol) are often used as a protocol that provides the function of the transport layer (layer 4). These are standard communication protocols used in Internet networks.
UDP supports a connectionless service. Therefore, UDP is not used to transfer a message. On the other hand, TCP supports a connection-oriented service. Therefore, TCP allows reliable data transfer. However, a transfer delay may occur and prioritized transfer is not allowed. Accordingly, it is difficult to use TCP for message transfer.
On the other hand, SCTP is a highly reliable communication protocol capable of performing congestion control. SCTP has, for example, the following characteristics.
(1) SCTP is a message-oriented communication protocol. That is, framing per message is enabled, avoiding missing the structure and order of a datagram.
(2) SCTP avoids duplicate transmission of a packet or the like and ensures reliability by validating a packet on the basis of a sequence number and checksum.
(3) SCTP is a multihoming-enabled communication protocol that allows one stream to have a plurality of communication paths, so SCTP has an outstanding fault recovery function.
In SCTP, a communication connection (association) is established between the transmission side and the reception side of a message to perform communication.
An association is different from a TCP connection in that a plurality of separate communication channels (streams) may be logically supported for one association internally.
All streams (for example, stream #01, stream #02, stream #03, and stream #04) in an association are independent of each other as illustrated in FIG. 4, but these streams are associated with a certain association. Each of the streams is given a stream number (for example, #01 to #04) and each stream number is encoded in an SCTP packet flowing through the association.
In multi-streaming, it is important that a blocked stream does not affect other streams in the association.
A stream is assigned for each user and is used when, for example, data of a call connection processing is transmitted or received.
As a technique related to SCTP, a method has been proposed in that a message with an urgency higher than urgencies of other communications is preferentially processed in a layer in which streams are controlled.
Another method has also been proposed in that data in communication between stations is transmitted depending on the priority.
Japanese Laid-open Patent Publication No. 2010-087990 and Japanese Laid-open Patent Publication No. 2011-044800 disclose related techniques.
The following non-patent documents also disclose related techniques: L. Ong et al., “An Introduction to the Stream Control Transmission Protocol (SCTP)”, online, Internet (URL: https://www.ietf.org/rfc/rfc3286.txt), May 2002 (searched on Feb. 21, 2012), R. Stewart et al., “Stream Control Transmission Protocol (SCTP) Specification Errata and Issues”, online, Internet (URL: https://www.ietf.org/rfc/rfc4460.txt), April 2006 (searched on Feb. 21, 2012), and R. Stewart, Ed., “Stream Control Transmission Protocol”, online, Internet (URL: https://www.ietf.org/rfc/rfc4960.txt), September 2007 (searched on Feb. 21, 2012).
Recently, with the spread of radio terminals such as mobile phones, the number of emergency telephone calls to emergency contact departments such as the police, ambulance, and fire departments from radio terminals is increasing year by year.
According to a research by the Tokyo Metropolitan Police Department, the number of emergency telephone calls to the police received from January to November 2010 in Japan is 8,491,285, which is greater than in the same period in 2009 by 236,958 (2.3%).
Of 8,491,285 emergency calls above, the number of calls from radio terminals is 5,593,241, which is more than half of the total.
In the case of, for example, a disaster or the like, radio base stations may be congested by general calls having a relatively low priority. Since communication that depends on the priority is not defined for communication using SCTP, a delay may occur in processing of emergency calls having a relatively high priority.